Blender built as a Python module, now how do I install and use it?
Okay, I think I got it.

cd to ~/blender-git/build_linux_bpy and run make install
this will install to a local path (~/blender-git/lib/linux_centos7_x86_64/python/lib/python3.7/site-packages/2.91)
setting WITH_INSTALL_PORTABLE=OFF on the command line before the call to make install had no effect
~/blender-git/build_linux_bpy/CMakeCache.txt defines the PYTHON_SITE_PACKAGES variable
I found this by running a search using grep -rnw . -e 'PYTHON_SITE_PACKAGES'
I set this PYTHON_SITE_PACKAGES to /usr/lib/python3/dist-packages
I chose that path from the list returned by python3 -m site
I can now import the bpy module from a python script

If anyone has a better way of doing this, let me know! Cheers.